When I got the error message, I pasted the freqz line into a session that I was using for work. It must have gotten broken somehow, because it works perfectly in a fresh session. The label does indeed say "Frequency (Hz)". Sorry about the confusion!

Maybe you can just go ahead and assume that when the FS argument is given, every calculation and label switches over to using Hertz rather than radians. That way we can move forward. This certainly seems like an easy bug to fix.

This is something I've run into before but not gotten around to yet. I'd recommend going even further to make the appearance closer to what users may expect: ditch the legend, labels on x and y axes for both plots.

Can someone with access to Matlab check whether the x axis label says "Normalized Frequency (rad)" vs "Frequency (Hz)" depending on whether the optional "fs" argument is given to freqz?